Yeah, squirrels are really problematic for mangoes, avocadoes, and lychees. In addition to being greedy little pigs, they are also wasteful, often taking only one bite, and leaving the fruit to rot, before moving to the next fruit, where they do the same.
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I sympathize with your friend. Lychees are a high value crop - if not monetarily, then just by virtue of their deliciousness. For starters, she will want to make sure and limit their access to the tree. If it's next to a house, that will be impossible. But if it's not, keep the branches trimmed back from any "access" points in adjacent trees or structure. Next, you can wrap the trunk with something very uncomfortable. Something like those wires they put out for birds. And as an extra layer of defense, a good electric fence is every farmer's best friend. You can actually buy twine that has metal ribbon in it, so it's not so expensive, and it's really easy to work with. If you can insulate the trunk from the wire, you could potentially run spirals of it, to hit them while they climb.
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And, of course, a dog that chases varmints is a good addition.